Carbon Reduction of Chromites in Bird River and Other Ores and Concentrates at 1200–1700°C

The equilibrium between Bird River chromite, and other chromites partly reduced by carbon and liquid metal is examined, and compared to that for Transvaal, Zimbabwe Dyke and Turkish minerals. The method used assumes that the activities of FeCr2O4 and MgCr2O4 are equal to their mole fraction after partial reduction. The equilibrium constant for the reduction is calculated as a function of temperature, CO pressure, and chrome to iron ratio remaining in the mineral, and is separated into a portion that involves conditions of reduction, equated to another containing only the activities of carbon, chromium and iron in the liquid metal reduced. For the metal, activities are obtained via Gibbs-Schuhmann integration of an expression for the carbon activity coefficient. It is found that after reduction of magnetite in the mineral, FeCr2O4 is reduced next, followed by MgCr2O4. In the case of the Bird River chromite, almost 90% of the chromium is present as the more readily reduced FeCr2O4, as compared to the other three ores where only 36–56% of the chromium is in the more readily reduced form.